1. Field of the Invention
The present invention relates to a light guide, a lighting device and a liquid crystal display device.
2. Description of the Related Art
For a front light of a reflective liquid crystal display device, a unit is used which is configured of a light source, an intermediate light guide, a light guide plate and a case for integrally holding them in which the inner side is formed to be reflective.
FIG. 13A is a perspective view illustrating a liquid crystal display device having this configuration, and FIG. 13B is a plan view illustrating the liquid crystal display device shown in FIG. 13A. The liquid crystal display device shown in these drawings is configured of a liquid crystal display unit 120 and a front light 110 disposed on the front side of the liquid crystal panel 120. The detail is not shown in the drawings, however, the liquid crystal display device 120 is a reflective liquid crystal display unit that reflects light incident from the front side thereof for display in which a liquid crystal layer is sandwiched between an upper substrate 121 and a lower substrate 122 disposed as facing each other. The alignment state of the liquid crystal layer is controlled to vary the condition of light transmission for display.
The front light 110 is configured to have a flat light guide plate 112, a bar-shaped intermediate light guide 113 disposed on a side end surface 112a of the light guide plate 112, and a light emitting device 115 disposed on one end surface part of the intermediate light guide 113 in which a prism form formed of a plurality of protrusions 114 having a wedge shape seen in cross section is formed on the top side of the light guide plate 112. Then, the front light 110 irradiates the light emitted from the light emitting device 115 to the side end surface 112a of the light guide plate 112 through the intermediate light guide 113 and introduces it into the light guide plate 112. By reflecting the light on the inner side of the top of the light guide plate 112 where the prism form is formed, the direction of light transmission is changed, and the light is irradiated toward the liquid crystal display unit 120 from the under side of the light guide plate 112 in the drawing. As this type of front light, that described in Patent Document 1 below is known.
[Patent Document 1]
Japanese Unexamined Published Patent Application 2000-11723
In the meantime, in portable electronic devices such as a personal digital assistant and a hand-held game machine, the time for driving a battery greatly influences the usability. On this account, for the liquid crystal display device used as a display part for them, a single light type front light having only a single light emitting device 115 such as the front light 110 shown in FIG. 13A is used in order to achieve the realization of low power consumption in the front light. More specifically, the light emitting device is omitted to realize low power consumption.
However, in this single light type front light, it is almost impossible to uniformly and brightly illuminate a large area of a display screen having a size of a few inches or greater by combining a thin light guide plate with a single light emitting device. More specifically, in the front light 110 shown in FIG. 13A, in the case of the configuration that the light emitting device 115 is disposed on one side, the incident light first needs to be uniformized in the longitudinal direction of the side end surface of the light guide plate 112 by the intermediate light guide 113 in order to uniformly guide the light from the light emitting device 115 to the light guide. However, since it is difficult to uniformize the incident light into the light guide plate 112 by the intermediate light guide 113, it is extremely difficult to obtain uniform outgoing light throughout the front side of the light guide plate 112, thus sometimes causing the visibility of the liquid crystal display device to be reduced.
As described above, the demand for the front light using a single light emitting device as a light source is increasing, but a front light capable of uniformly and brightly illuminating a large area is not realized yet.